1. Field of the Invention
The present invention relates to an output undervoltage warning method for identifying a cause for insufficiency of an output voltage reduced to or below an undervoltage set value, an output undervoltage warning circuit used for executing the method, and a switching power supply.
2. Description of the Related Art
The Patent Literature 1 proposed a power supply capable of displaying an operation state thereof on display devices and thereby monitoring it. The power supply displays whether or not output terminals are in a short-circuit state on two display devices. As a proposal for dealing with the problem that the insufficiency of an output voltage of the power supply may result in the malfunctioning of an electronic device and the like, an undervoltage warning circuit for a warning of the insufficiency of the output voltage of the power supply is additionally provided. FIG. 5 shows a conventional undervoltage warning circuit 12. An output voltage of a switching power supply is applied to a terminal 12i of the output undervoltage warning circuit 12. An input circuit 12a divides the output voltage by resistances R2 and R4, and supplies a divided voltage Va to a plus input unit (+) of a comparing circuit 12b and supplies a constant reverse conducting voltage Vb generated by a resistance R1 and a voltage regulating diode ZD to a minus input unit (−) of the comparing circuit 12b. A variation of the voltage with respect to the plus input unit (+) of the comparing circuit 12b is in proportion to a variation of the output voltage. The voltage with respect to the minus input unit (−) of the comparing circuit 12b corresponds to an undervoltage set value.
When the output voltage is normal, Va>Vb is obtained Therefore, an output voltage Vc of the comparing circuit 12b is at a high level, and a LED element 12c is turned off and a transistor 12d is conducted. When the transistor 12d is conducted, a warning signal that indicates that the output voltage is normal is outputted to output terminals 12g via a photocoupler 12e. The normal output voltage results in the combination of the LED element 12c being turned off and the transistor 12d being conducted. When the normal state shifts to an overcurrent state, thereby reducing the output voltage applied to the terminal 12i to result in V<Vb, the output voltage Vc of the comparing circuit 12b is at a low level, and the LED element 12c is turned on and the output transistor 12d is non-conducted. The combination of the LED element 12c being turned on and the transistor 12d being non-conducted represents a load abnormality. When a life of a switching power supply 10 has run out, which leads the output voltage to reduce as Va<Vb, the output voltage Vc of the comparing circuit 12c is at the low level. As a result, the LED element 2c is turned on, and the output transistor 12d is non-conducted. The combination of the LED element 12c being turned on and the transistor 12d being non-conducted represents a power supply abnormality. The output voltage of the power supply is reduced to zero when overvoltage, overheating, halt of an alternate current input with respect to the power supply or the like occurs, the LED element 12c is turned off, and the transistor 12d is non-conducted. Any combination of the turned-off LED and the non-conducted transistor represents the power supply abnormality. Among the foregoing combinations, the combinations of the state of the LED element 12c and the state of the output transistor 12d are identical in the case of the output undervoltage insufficiency due to the overcurrent and the exhausted life of the power supply. Therefore, it is not possible to discriminate if the output undervoltage is caused by the load abnormality or the power supply abnormality.
[Patent Document 1] No. 2004-320825 of the Publication of the Unexamined Japanese Patent Applications